1. Field of the Invention
The present invention relates to the field of voltage converters and in particular to a charge pump voltage converter.
2. Description of the Related Art
Many electrical devices require power supplied at a stable voltage different than that provided by a primary power source. In many applications, the primary power source is a battery. Often the electronic devices require voltages that are between 1 and 2 times the voltage provided by the battery. An additional requirement is that the voltage provided be relatively stable. A low voltage can result in the powered devices failing to operate at all or at a reduced performance level. Steady overvoltage can reduce the life of the devices or permanently damage the devices. Spikes or transients in voltage can also disrupt device operation and cause damage.
One difficulty with batteries is that many batteries do not provide a stable output voltage. The output voltages of many batteries decrease as the batteries are used and as the batteries age. The voltages can also vary depending on how heavily the batteries are loaded. Certain batteries also vary in output voltage with variations in temperature. Even under conditions where the battery voltage is not varying, the battery may provide power at a different voltage than that required by user devices.
Charge pumps are known circuits that effectively transfer electrical charge back and forth between storage components to generate an output voltage different from an input voltage. Charge pumps with a xe2x80x9cbuckxe2x80x9d feature are effectively voltage limiters. If the input voltage exceeds a threshold value, the charge pump xe2x80x9cbucksxe2x80x9d the overvoltage away from the load. However, in a charge pump circuit, the charge is not simply shunted to ground or another load as in, for example, zener diode circuits. A charge pump temporarily stores the charge redirected from the load, typically in a capacitive element. This charge stored in the capacitor is then typically delivered to the load at a later time. Zener diodes are effective at clamping voltages above a certain threshold; however, by simply shunting the current away from the load, the current is typically not available for use. This results in wasted power. It will be appreciated that wasting power in a device with limited battery capacity is preferably avoided.
In a xe2x80x9cboostxe2x80x9d operation, a charge pump accumulates charge to be able to provide a greater voltage to the load than is provided by the input voltage. A charge pump in boost operation typically sequentially charges at least one capacitor connected in parallel with the power source and then selectively interconnects the capacitor(s) in series with power source to increase the available voltage for delivery to the load. Typical charge pump circuits double or triple the input voltage minus some switching and other loses. Again, it will be appreciated that minimizing loses from a limited power source such as a battery is desirable.
Charge pump circuits are often used in consumer electronics, such as PDAs, cell phones, and the like. Thus, it will be appreciated that simplicity and low cost are highly desirably. With potential markets in the millions of units, a reduction in cost of only a few cents can add up to significant savings and increased profits for the manufacturers and sellers. An additional design goal is to reduce size and weight of the devices. Reduced size and weight increases the convenience of an appliance to the consumer and increases the marketability of the appliance. Many known charge pump designs employ multiple operating modes that increase circuit complexity and cost of the charge pumps. Multiple operating modes also generally lead to voltage transients upon switching between the multiple modes, which again can damage powered devices.
An additional problem with the charge pump circuits is that the conversion efficiency declines very quickly when the output voltage is less than twice the input voltage. The general rule for any charge pump is that input current will always be twice the output current when the circuit is in equilibrium. Current is drawn from the battery on every clock cycle; however, it is only supplied to the load every other clock cycle. Thus, the instantaneous current is the same in the input and output sides, but the time average current for the input is twice the output. Since efficiency is the ratio of power out divided by power in, and input current is always twice output current, the maximum theoretical efficiency can be calculated as follows:
Eff=Pout/Pin 
Eff=(Ioutxc3x97Vout)/(Iinxc3x97Vin) 
Eff=(Ioutxc3x97Vout)/(2xc3x97Ioutxc3x97Vin) 
Eff=(Vout/(2xc3x97Vin))xc3x97(100%) 
Therefore:
Eff=100% max when Vout=2 Vin
Eff=50% max when Vout=Vin
Eff=25% max when Vout=xc2xd Vin
In practical circuits, it is reasonable to expect 10% losses in the converter because of resistive losses in switching components and junction drops. The actual predictions made by multiplying the theoretical predictions above by 90% are 90%, 45% and 22%, respectively.
One reason for the low efficiency numbers when Vout is  less than 2 Vin is that the charge current for the transfer capacitor Cx flows from the battery directly to ground without imparting its full energy on Cx, (i.e., Cx is not charged to the battery""s full potential). Thus, the voltage difference between the available battery potential and the voltage needed to charge Cx to obtain the desired output voltage is available, but is not utilized.
Many real world battery-powered applications that include multiple loads with different power requirements and meet the criteria for using a secondary load output from the charge pump. For example, cellular telephones and handheld computers (PDAs) are typically powered by single cell Lithium-ion or triple cell NiCad batteries having terminal voltages that range from 5.6 volts at full charge to 3.0 volts at cutoff. A common requirement in these products is to provide one regulated output voltage in the 5-volt to 3.3-volt range and to provide a second regulated output voltage in the 2.5-volt 1.5-volt range.
A very specific application in the cellular phone product area is driving a combination of white and green LEDs that light a color display and a keypad. White LEDs are needed to provide good color from a liquid crystal thin film transistor (TFT) display. These white LEDs typically have forward voltage drops of 3.6V and draw approximately 20 mA each. Therefore. the white LEDs require a buck/boost voltage converter to operate from the normal 5.6-volt to 3.0-volt battery potential. An additional requirement for the white LEDs is that all the LEDs generate approximately the same light intensity in order to achieve uniform lighting in the display.
The cell phone also uses a lighted key pad, but this can use lower cost and lower voltage green LEDs. Uniformity of lighting and color trueness of the keypad is less of a concern than with the display. Green LEDs operate at 2.0 volts at 10 mA drive levels, thus using less power per LED (≈20mW) than white LEDs (≈72mW). However, because the electrical requirements of green and white LEDs differ, two separate circuits are typically required to enable to advantages of using green and white LEDs.
In order to maintain a constant and consistent light output, multiple LEDs require a constant current rather than constant voltage. One method of driving LEDs is to use a constant voltage source with current limiting ballast resistors in series with the LEDs to sense and/or control forward current. Multiple LEDs can be driven in parallel or in series. If in series, only one series resistor is required for the LEDs in that branch, however the supply voltage must be high enough to support the sum of the forward voltages of the LEDs. Unfortunately, the voltage required for two or more LEDs is higher than readily achievable with a switched capacitor charge pump fed by a typical battery.
When multiple LEDs are driven in parallel, the supply voltage only needs to be in the 4V range, which is easily achievable with a charge pump operating from a 3-volt battery. In the parallel case, each LED has its own series resistor to control and balance its current. However, this approach has two weaknesses:
1) Current matching among the LEDs is needed to insure equal light output. Individual LEDs will have different forward voltage drops at equal drive currents. Because of this, the value of a series ballast resistor must be fairly high to control current sharing without undertaking the significant time and expense of testing and selecting LEDs for minimal variations in forward voltage. Typically, dropping at least one-fourth of the forward voltage of the LEDs would be needed to maintain less than 10% current variation among the multiple LEDs. With a primary supply voltage of 3.0 volts providing power to a charge pump powering white LEDs with 3.6-volt forward voltage drops, 0.9 volts would typically be needed across the series resistor to achieve less than 10% current variation. This would result in a 20% loss because one-fifth of the total supply voltage is used in the ballast resistor and energy is lost to resistive heating rather being used for the desired light production.
2) The product would be burdened with the extra space and cost of the series resistors.
Thus, from the foregoing it will be appreciated that there is a need for an efficient charge pump that provides both buck and boost operations and that provides an output voltage that is regulated to provide a stable output voltage even in the presence of variations in an input voltage. A need also exists for a regulating charge pump of simple design that avoids the cost and complexity of multiple operating modes. A need also exists for a charge pump that avoids switching between multiple operation modes and that minimizes switching transients. Furthermore, a need exists for a buck/boost capable charge pump that can provide regulated outputs at different voltage levels with a single circuit. Advantageously, such a multiple output regulating charge pump operates with improved efficiency. Moreover, a need exists for a single circuit that provides multiple voltage regulated outputs and that also regulates the current in multiple branches of at least one of the outputs so as to facilitate powering LEDs in a highly efficient and balanced manner.
One aspect of the present invention solves these and other problems by providing a single mode buck/boost charge pump that provides a regulated constant output voltage between zero and twice an input voltage without changing control modes or interrupting circuit operation when the input voltage falls below or rises above a set output voltage. In one embodiment, a single mode buck/boost charge pump is adapted to power a plurality of separate loads in a highly efficient manner. In another embodiment, a single mode buck/boost charge pump is a combination current regulator and multiple output regulating charge pump adapted for driving LEDs in a highly efficient and balanced manner.
In one aspect of the present invention, a regulating charge pump provides buck and boost operation in a single operating mode wherein the charge pump provides an output voltage that is a multiple of an independent reference voltage and wherein a charge storage component is charged by a regulated variable current supply. In one embodiment, the variable current supply is regulated with respect to the reference voltage and the output voltage and the charge storage component is alternately charged by the regulated variable current supply and connected in series with the output. In certain embodiments, the charge storage component is inhibited from being charged when connected in series with the output.
In certain embodiments, the reference voltage is a fixed voltage, and in alternative embodiments, the reference voltage is selectable from among a plurality of voltage values.
In another aspect of the present invention, a regulating charge pump receives a supply voltage and provides a regulated output voltage. The charge pump comprises a charge storage component, a plurality of switches interconnecting the charge storage component and the supply voltage, a switch timing control that regulates the states of the plurality of switches, a reference voltage source, an error amplifier connected to the reference voltage source and the regulated output, and a variable current supply that receives control signals from the error amplifier and provides regulated current to the charge storage component in response to the output voltage, wherein the output voltage is regulated with respect to the reference voltage source. In particular embodiments, the switch timing control alternately connects the charge storage component to the variable current supply and in series with the regulated output. In certain embodiments thereof, the switch timing control inhibits connecting the charge storage component to the variable current source and the output simultaneously. The switch timing control operates in a periodic fashion.
In certain embodiments, the reference voltage is a fixed voltage. In alternative embodiments, the reference voltage is selectable from among a plurality of voltage values. In other embodiments, the error amplifier comprises a feedback network and in certain embodiments thereof, the feedback network comprises a voltage divider connected to the regulated output.
In a further aspect of the present invention, a method provides a stable output voltage. The method comprises providing an input voltage and providing a reference voltage. The method sequentially charges a charge storage component via a regulated variable current source and connects the charge storage component in series with the input voltage so as to generate the output voltage. The method monitors the output voltage and regulates the charging of the charge storage component such that the output voltage is a multiple of the reference voltage.
In one embodiment, the present invention is useful in charge pump applications where a supply voltage, Vin, is higher than a minimum supply voltage needed to provide the output voltage Vout. Thus, the charge component is not charged to a maximum value that it can reach. The difference between the minimum supply voltage and the maximum voltage on the charge storage component is used to generate a second voltage output from the circuit. The second voltage output is supplied to a second, separate load. In this aspect, the present invention is able to supply different multiple regulated outputs from a single input voltage. In certain embodiments, the input voltage is lower than one output voltage and higher than the other output voltage.
For example, at a minimum battery voltage of 3.0V, white LEDs require a 0.6-volt boost, plus about 200 mV to implement a constant current driver. Thus, the minimum output voltage provided to white LEDs must be about 3.9 volts to account for other circuit losses. The total boost required from a charge pump is then 3.9xe2x88x923.0=0.9 volts. Since the minimum battery voltage is 3.0 volts and the 0.9-volt boost must appear across the charge transfer component while it is being charged, the difference of 2.1 volts (3.0xe2x88x920.9) is available to drive the second load. It is common to use four green LEDs operating at 10 mA to light the keypad. The remaining 2.1 volts is adequate to do this with 100 mV left over for circuit losses. The total current required by two white LEDs is approximately the same as required by the four green LEDs. This is advantageous because virtually all of the unused energy from the charge pump can be diverted to the green LEDs. In addition, since both the white LEDs and the green LEDs are typically turned on at the same time, it is advantageous to share the same charge pump circuit.
In one aspect of the present invention, a charge pump receives a supply voltage wherein the charge pump provides multiple regulated outputs. In one particular embodiment, the multiple regulated outputs are at different voltages, and at least one of the multiple outputs is regulated at a voltage different than the supply voltage. In certain embodiments, the outputs are regulated independently with respect to input voltage.
In another aspect of the present invention, at least one of the outputs is regulated with respect to a parameter of a load connected to the at least one output. In one particular embodiment, the parameter of the load corresponds to an output node of the load. In another embodiment, regulating the at least one output with respect to the parameter of the load automatically compensates the at least one output for variations in the parameter of the load. In this embodiment, the variations in the parameter of the load include variations due to temperature change.
In a further aspect of the present invention, a multiple output regulating charge pump receives a supply voltage. The charge pump comprises a charge storage component, a plurality of switches interconnecting the charge storage component and the supply voltage, a switch timing control that regulates the state of the plurality of switches, a reference voltage source, and a feedback circuit that provides regulated current to the charge storage component in response to the output voltage, wherein the output voltage is regulated with respect to the reference voltage source. In certain embodiments, the multiple outputs provide regulated voltages to at least a first load and a second load. In a particular embodiment, the output voltage is further regulated with respect to at least one of the first load and the second load. In an embodiment thereof, the output voltage is regulated with respect to an output node of at least one of the first load and the second load.
In yet another aspect of the present invention, the switch timing control operates the switches so as to alternately charge and discharge the charge storage component. In one embodiment, charging the charge storage component comprises connecting the charge storage component in series with the supply voltage and the second load, and discharging the charge storage component comprises connecting the charge storage component in series with the supply voltage and the first load. In a certain embodiment, current is provided to the first load as the charge storage component is discharged and is provided to the second load as the charge storage component is charged. In another embodiment, the switch timing control operates the switches so as to inhibit having the charge storage component connected in series with the supply voltage and both the first and the second loads simultaneously.
In particular embodiments of the invention, the feedback circuit comprises a variable current source and an error amplifier and the voltage reference provides a fixed reference voltage. In a further embodiment, the reference voltage is selectable among a plurality of reference voltage values.
In one embodiment, a multiple output regulated charge pump is combined with constant current sinks for multiple white LEDs to provide an LED driver and a load current regulator with higher efficiency. This also results in a lower component count. In addition, a greater accuracy can be obtained for cell phone and PDA applications that must operate from batteries having voltages that range from 3.0 volts to 5.6 volts. The device is scaleable to different quantities of LEDs by simply adding a current sink for each additional white LED in the application. The load current regulator is capable of maintaining less than 10% current variation among the white LEDs with only a 300 mV overhead and eliminates the need for ballast resistors in the load.
In one aspect of the present invention, a multiple output regulating charge pump receives a supply voltage and provides at least a first regulated output and a second regulated output. The first regulated output has a voltage that can be regulated at a level different than the voltage of the supply, and the current provided to a load by the first output voltage is actively current regulated. In certain embodiments, the first output is voltage regulated with respect to an output node of the load connected to the first output, thereby automatically compensating for variations in load characteristics.
One aspect of the present invention is a charge pump with a charge storage component and a plurality of switches connected to the charge storage component under control of a switch timing control circuit. The switch timing control circuit controls the switches to sequentially connect the charge storage component to the supply in series with the first output and then in series with the second output. The charge storage component is alternately charged when connected in series with the second output and discharged when connected in series with the first output so as to provide the first regulated output voltage. The switch timing control operates to prevent the charge storage component being connected to both the first and the second outputs simultaneously. In certain aspects of the invention, the switch timing control receives timing signals from an oscillator such that the switch timing control circuit operates to open and close the switches in a periodic fashion.
In another aspect of the present invention, a current is supplied to a load connected to the second output when the charge storage component is being charged and at least the first output is voltage and current regulated so as to provide substantially equal currents to multiple branches of the load connected to the first output.
Another aspect of the invention is a load current regulator that regulates the current provided to the load connected to the first output. In particular, the load current regulator regulates the current among the multiple branches of the load connected to the first output such that the current in each of the branches of the load is substantially equal.
In certain embodiments, the load current regulator comprises a plurality of transistors arranged in a current mirror configuration and the load connected to the at least first output comprises a light emitting diode.
A further aspect of the present invention is a regulating charge pump that receives a supply voltage and that provides regulated voltages to at least two loads. The charge pump comprises a charge storage component, a variable current source, an error amplifier that receives feedback from at least one of the loads and provides control signals to the variable current source, and a plurality of switches that interconnect the supply, the charge storage component, the variable current source, the error amplifier, and the at least two loads. A switch timing control circuit controls the operation of the switches such that the variable current source can supply current to the charge storage component and directly to at least one of the loads. A load current regulator is connected to at least one of the loads such that currents within multiple branches of the load are actively balanced.
In certain embodiments, the error amplifier receives feedback from an output node of the at least one load. The switch timing control circuit operates the switches such that the charge pump alternately provides regulated voltage to a first load as the charge storage component discharges and provides regulated voltage to a second load as the charge storage component is charged.
In certain embodiments in accordance with the foregoing aspects of the present invention, the charge pump includes a switch timing control circuit that operates the switches in a periodic manner. The switch timing control circuit prevents all the switches from being turned on at the same time. The load current regulator comprises a plurality of transistors arranged in a current mirror configuration.
The foregoing aspects of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.